I'm not going to speak for everybody who prefers D2 and classic EQ, but what I enjoyed was the feeling of having earned what you had. That's what makes what might otherwise be called tedious, enjoyable to me. You 'work' hard and you get your reward at the end. The more effort required to get the reward, the more rewarding it feels. Blizzard makes everything feel unrewarding to me in all their current generation games in order to cater to the casuals.
There is a second factor involved though: it has to be challenging. In both games, the best experience was (often) found in zones where death was a real possibility, and both had significant death penalties to make you pay attention. I mean just look at flappy bird: why the fuck did that game make a ridiculous amount of money? It's the most simple game imaginable, but ridiculously hard; not that flappy bird is some brilliant example of game design or anything, but it did make $50kper dayfor the guy.
Now D3 inferno was 'hard' at launch, and few liked it that way. So why why didn't it work? I would argue that it was because it was simply tuned to one-shot everybody which made death completely unavoidable in many situations regardless of how well you played-- the game simply wasn't designed with players avoiding all damage output in mind. Also, going offensive stats generally was suicide unless you had some immunity ability (smokescreen) or one-shot prevention ability (force armor), which made class balance absurdly lopsided. With the game now, going offensive stats grants some defensive ability in the form of lifesteal, so the game favors offensive stats without making defensive stats useless, which is good.
D2 vs D3 and EQ vs WoW is just hardcores vs casuals. I would argue that attempting to appeal to both audiences at once is a mistake and that studios should just target one or the other. Blizzard keeps trending toward the casual end of the spectrum which is fine, but it would be nice if games for people like me were still being made so I don't have to play 15 year old games to get what I want. (and yes, I still play classic EQ)